A dynamic compressor is one of the most commonly used means for gas compression in industry. Common dynamic compressors include centrifugal and axial flow compressors. During operation, a dynamic compressor can become unstable due to changes in various operating conditions (such as, gas composition, flow rate, speed, pressure, etc.), causing rapid pulsations in flow. This phenomenon is called surge. Surge conditions occur in a dynamic compressor when the inlet flow is reduced to the extent that the compressor, at a given speed, can no longer operate at the existing head. At this point, a momentary reversal in flow occurs resulting in a drop in head. Normal compression resumes and the phenomenon repeats. This surge phenomenon is highly undesirable since the resulting noise, vibration, and over heating can lead to mechanical damage of the compressor, associated instrumentation, and piping. Various surge avoidance schemes are used with dynamic compressors to prevent surge.
A typical surge avoidance scheme avoids surge by activating a bypass valve to redirect flow around the compressor when the compressor approaches a surge condition. The compressor approaches a surge condition when a compressor operating point approaches a preset compressor surge limit. When the compressor operating point approaches the surge limit, the bypass valve opens to redirect a portion of gas from the discharge side of the compressor to the suction side, thereby reducing the head across the compressor. The reduced head increases flow through the compressor, thereby preventing surge.
The compressor surge limit is typically obtained from characteristic set point curves (“compressor maps”) provided by the compressor manufacturer. These compressor maps define a zone for stable operation of the compressor. Common compressor maps define a surge line of the compressor as a function of three variables—head, flow, and the speed. These variables are typically determined based on sensor readings of various operating conditions of the compressor. The compressor operating point at any particular time may be plotted on the compressor map using measured/determined values of head, flow, and speed, at that time. When the operating point approaches the surge line, a surge condition is detected, and the bypass valve is opened to redirect flow from the discharge side to the suction side or simply pass it off through a blow-off line to prevent surge. A defect in a sensor used to determine head, flow, or speed, may cause a fault in the surge avoidance system. Typically, a compressor may be shut down in response to a faulty surge avoidance system to avoid surge of the compressor. Unplanned shut down of the compressor may affect down stream operations and productivity.
Various attempts have been made to develop suitable surge avoidance techniques to minimize compressed gas bypass while maintaining the compressor in a surge free state. U.S. Pat. No. 4,861,233 (the '233 patent) issued to Dziubakowski et al. on Aug. 29, 1989 describes a dynamic compressor surge control system which provides anti-surge protection in proportion to the magnitude of the anticipated surge condition. The control system of the '233 patent anticipates a surge condition in advance of the surge line in the compressor map by establishing a surge control line, offset from the surge line, using a control signal. When the operating point of the compressor in the '233 patent approaches the surge control line, anti-surge measures are initiated. The control signal of the '233 patent, which establishes the surge control line, is based on a control variable other that the one used to establish the surge line. The offset of the surge control line from the surge line, in the '233 patent, varies depending upon the rate of change of the control variable and provides an indication of the magnitude of the anticipated surge condition.
While the control system of the '233 patent may initiate surge avoidance measures depending upon the magnitude of the anticipated surge condition, the control system still relies on trouble free performance of multiple sensors to measure different parameters that establish the operating point. A defective sensor may indicate an erroneous value of one of measured parameters, causing faulty operation of the surge control mechanism. In response to a faulty operation of the surge control mechanism, the compressor may need to be shut down for sensor repair/replacement, or operated in an excessively conservative manner to ensure that that the compressor does not surge. Unplanned shut down, or a significant reduction in compressor performance, may adversely effect upstream or downstream equipment/operations and productivity.